With development of power electronic technology, various non-linear power electronic instruments are widely used in electric power system, industry (particularly, metallurgy, steel, chemical industry or the like), traffic, building automation and family, and are increased in capability and control manners, which enhances waveform distortion of the voltage and the current, and thus harmonic pollution in the electric power network is increasingly severe. For reactive power compensation and harmonic suppression in the electric power network, a passive power filter (PPF) is used both at home and abroad, which has a simple structure, low cost and good filtering effect on specific harmonic. For example, on a power supplying network of industry such as metallurgy and metal, a PPF is mounted, which has the capacity from several MVAs to dozens of MVAs. The PPF improves the electric power quality, but the filtering effect thereof is greatly affected by the resistance and the parameters of the electric power network and it tends to resonate with the electric power network impedance which influences the safety of power supply. In order to overcome the disadvantages of the PPF, an active power filter (APF) becomes a hotspot of research and application. Compared with the PPF, the APF has the following advantages and disadvantages:
1. The APF has various functions of compensation, which can compensate not only each subharmonic, but also dynamically compensate reactive power and negative sequence current or the like;
2. The filtering performance is not affected by the impedance of the electric power network, and will not resonate with the electric power network impedance;
3. The harmonic compensation performance is not affected by the change of frequency of the electric power network;
4. Dynamic harmonic suppressing is achieved, in which changes of the frequency and the amount of the harmonic can be quickly responded;
5. Since the device has limited output capability, even if the amount of the harmonic increase, the no over-loading phenomenon will occur;
6. With good cost performance, APF can achieve several harmonic controls;
7. One harmonic resource can be controlled at a time and several harmonic resources can be as well.
Due to the advantages and the characteristics of the APF, the APF becomes popular among users. Limited by current power electronic switch means, APF is generally used in harmonic control of a low voltage power supply system. For middle or high voltage, large capability harmonic control, a main circuit of APF generally requires series connection, parallel connection, multiple types and multilevel technology of a switch means. If the series connection and parallel connection of the switch means are used, problems of dynamic voltage balancing and current balancing are required to be solved. A transformer applied in a multiple technology has saturation and nonlinearity, which brings problems of complex control and difficult protection. Thus, improving capability of the APF by multilevel technology is an important research direction.
In 1996, F. Z. Peng and J. S. Lai et al. proposed a topological structure of a chain circuit multiple inverter in “A Multilevel voltage-source inverter with separate DC source for Static Var Generation” (IEEE Transactions on Industry Applications, 1996, 32(5):1130-1138.) The topological structure is used in an APF to constitute a chain circuit active power filter to significantly improve capability and withstand voltage level of the APF, and it is not required to increase a transformer to use in harmonic compensation of a middle or high voltage, large capability, nonlinearity laden, which has a wide application foreground.
A main circuit of a chain circuit APF is a chain circuit multilevel inverter. Each phase constitutes an independent chain, and the inverter is composed of multiple link units with the same structure. Each link unit can be composed of a single-phase full-bridge invert which can output three levels and control units thereof. With increasing levels of the link units, the capability and dielectric strength of the device increases. However, use of multiple switch means may increase probability of fault, thus it is necessary to increase redundant link units. If the number of the link units is less than or equal to that of the redundant link units, the link unit with fault can be bypassed. Then the device can continue to operate, which improves availability and reliability of the device. Therefore, there is an urgent need to design a simple economic reliable bypass structure.
Chinese patent 201010624231.1 and Chinese patent 201020700497.5 provide a chain circuit STATCOM link unit bypass structure with a mechanical switch, in which a special mechanical switch is applied as a bypass structure of a link unit. The circuit structure is simple and includes little elements and is easy to operate. Chinese patent 201010261610.9 provides a bypass structure where a contactor is utilized as a link unit of a transducer. Chinese patent 200520050490.2, 200810113842.2 and 200810113844.1 use controllable silicon as a bypass structure of the link unit. The above bypass structures all add a link unit bypass by adding a power means, which increases cost and volume. Furthermore, since a contactor or controllable silicon is used as an actuating means of the bypass, achieving the function of bypass takes more time.